MODEL STRUCTURES FOR MCM-41 MATERIALS - A MOLECULAR-DYNAMICS SIMULATION

Model structures for MCM-41 materials have been obtained using the classical molecular dynamics simulation technique with effective interaction potentials. A series of MCM-41 models with various lattice constants and wall thicknesses have been generated and analyzed. The density of T-sites, concentration of silanols, distribution of ring sizes, and the X-ray diffraction patterns were calculated for each structure with the latter compared to the experimental results. The results indicate (i) the calculated X-ray diffraction pattern of the greater than or equal to 10 Angstrom wall amorphous silica models are in excellent agreement with those observed experimentally, (ii) structures with wall thicknesses less than 7 Angstrom are most likely unstable and give rise to X-ray diffraction patterns inconsistent with experiment, (iii) structures formed with smaller micelle templates require thicker walls to achieve thermodynamic stability, and (iv) models whose calculated X-ray diffraction patterns are consistent with observed patterns have I7-28% of the silicon as silanols, in good agreement with the 8-27% reported from NMR.